1. Field of the Invention
This application claims priority from German Patent Application No. 298 22 472.0 filed Dec. 18, 1998.
2. Description of the Prior Art
A swivel joint as described above is well-known in the art. The prior art swivel joint comprises two joint members which may be pivotably connected to one another by an axle. The one joint member may hereby be linked directly to the one vehicle or vehicle part, whereas the other joint member may be connected to the vehicle or the vehicle part either directly by being pivotable around a horizontal running axle or indirectly via an intermediate link. Thanks to this axle, allowance is made for the fact that the swivel joint has to be able to yield to the nodding movements of the two vehicles relative to one another. According to a first embodiment, these swivel joints are provided on either side of the longitudinal axis of the vehicle or of the swivel joint with one hydraulic cylinder, whereas the hydraulic cylinders communicate on one hand with the one joint member and on the other hand with the vehicle. The damping members are hereby in angular alignment relative to the central longitudinal axis of the vehicles.
In another embodiment, a hydraulic cylinder with double-action is provided. It is fastened on the swivel joint across the longitudinal axis.
The electric drive of such a hydraulic system is known; an electrically triggerable proportional pressure control valve is hereby provided and is arranged between the force side and the suction side of the double-acting hydraulic cylinder or between the force side and the suction side of the two single-acting hydraulic cylinders. Such a proportional pressure control valve increases the flow resistance of the hydraulic medium. The consequence thereof isxe2x80x94as already explainedxe2x80x94that the buckling motion of the joint is damped, particularly in a curve. Such a damping of the motion of rotation of the two joint members relative to one another is absolutely necessary, since otherwise the vehicle is hardly controllable. Such articulated vehicles are often designed as so-called pushers; a pusher is a vehicle whose last axle is driven. In case the hydraulic system fails to work, i.e. in case the motion of rotation of the two joint members relative to one another is not being damped, the vehicle may, under higher velocity, swerve at the least steering maneuvre, since the rear vehicle pushes the front vehicle aside via the lateral axis. The same occurs when driving a straight course when the road has rail grooves. In this case, the oscillations of the vehicle may build up. That means that the hydraulic system particularly fails to work when the electrically triggerable proportional pressure control valve has broken down. This is the case when the power supply of the vehicle has been interrupted since the control of the proportional pressure control valve occurs via electric signals. The electric drive and with it the degree of damping of this proportional pressure control valve depends on the state of the vehicle, whereas influences such as for example the speed of the vehicle and the position of the front car of the vehicle relative to the rear car of the vehicle are taken into account. If, consequently, the hydraulic system breaks down at higher speeds, because of an interruption in the electric circuit for example, it has to be guaranteed that the vehicle may at least be slowed down and head for the nearest repair shop at a considerably reduced speed without incurring any risk.
The object of the present invention is therefor to develop a hydraulic system of the type mentioned above that guarantees the controllability of the vehicle even in case the electric installation of a vehicle and with it the damping effect on the motion of rotation of the two joint members of a joint relative to one another break down due to the failure of the proportional pressure control valve.
The solution of this object is to provide in the conduit between the suction side and the force side of the damping device a mechanical pressure control valve in addition to the proportional pressure control valve, a multiple-way valve being provided for the alternative drive of the pressure control valve or of the proportional pressure control valve. Such a pressure control valve is designed in such a way that it may be adjusted to a predetermined pressure drop. That means that the mechanical pressure control valve may be adjusted to a determined minimum pressure which determinesxe2x80x94as already explainedxe2x80x94the degree of the damping.
According to two embodiments, the mechanical pressure control valve may either be arranged in parallel or in series relative to the electrically working proportional pressure control valve in the conduit between the suction side and the force side of the damping device.
The hydraulic damping device comprises more particularly a double-acting piston-cylinder arrangement or, according to a further embodiment, two piston-cylinder arrangements. Such a double-acting piston-cylinder arrangement installed across the longitudinal direction of the vehicle has the advantage of being more compact from a constructional point of view.
In order to guarantee that, when the proportional pressure control valve is not working any longer and is consequently open, the hydraulic path is led through the mechanical pressure control valve, a multiple-way valve and, more particularly a 3/2-way valve in case of a connection in parallel of proportional pressure control valve and mechanical pressure control valve or a 2/2-way valve in case of a connection of the pressure control valves in series.
Other advantageous features are described in the claims.
Another subject matter of the invention is a joint of an articulated vehicle with a hydraulic system as described above.
The invention is explained in more details according to the drawing.